Light emitting diodes (LEDs) and in particular high intensity and medium intensity LED strings are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including, but not limited to, backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as monitors. In a large monitor the LEDs are typically supplied in one or more strings of serially connected LEDs, thus sharing a common current.
In order to supply a white backlight for the monitor, one of two basic techniques are commonly used. In a first technique one or more strings of “white” LEDs are utilized, the white LEDs typically comprising a blue LED with a phosphor which absorbs the blue light emitted by the LED and emits a white light. In a second technique, one or more individual strings of colored LEDs are placed in proximity so that in combination their light is seen as a white light. Often, two strings of green LEDs are utilized to balance one string each of red and blue LEDs.
In either of the two techniques, the strings of LEDs are in one embodiment located at one end or one side of the monitor, the light being diffused to appear behind the LCD by a diffuser. In another embodiment the LEDs are located directly behind the LCD, the light being diffused by a diffuser so as to avoid the appearance of hot spots. In the case of colored LEDs, a further mixer is required, which may be part of the diffuser, to ensure that the light of the colored LEDs are not viewed separately, but are rather mixed to give a white light. The white point of the light is an important factor to control, and much effort in design and manufacturing is centered on the need for a controlled white point.
Each of the colored LED strings is typically controlled by both amplitude modulation (AM) and pulse density modulation (PDM), typically by pulse width modulation (PWM), PWM being a particular embodiment of PDM, to achieve an overall fixed perceived luminance and white point. AM is typically used to set the white point produced by the disparate colored LED strings by setting the constant current flow through the LED strings to a value determined as part of a white point calibration process and PWM is typically used to variably control the overall luminance, or brightness, of the monitor without affecting the white point balance. Thus the current, when pulsed on, is held constant to maintain the white point produced by the combination of disparate colored LED strings, and the PWM duty cycle, i.e. the percentage of the PWM period for which the PWM signal is active, is controlled to dim or brighten the backlight by adjusting the average current over time. The PWM duty cycle of each color is further modified to maintain the white point, preferably responsive to a color sensor. It is to be noted that different colored LEDs age, or reduce their luminance as a function of current, at different rates and thus the PWM duty cycle of each color must be modified over time to maintain the white point. Thus, LED strings of various colors, which may be controlled by a single controller, will over time typically exhibit differing PWM duty cycles.
Each of the disparate colored LED strings has a voltage requirement associated with the forward voltage drop of the LEDs and the number of LEDs in the LED string. In the event that multiple LED strings of each color are used, the voltage drop across strings of the same color having the same number of LEDs per string may also vary due to manufacturing tolerances and temperature differences. Ideally, separate power sources are supplied for each LED string, the power sources being adapted to adjust their voltage output to be in line with voltage drop across the associated LED string. Such a large plurality of power sources effectively minimizes excess power dissipation; however the requirement for a large plurality of power sources is costly.
An alternative solution, which reduces the number of power sources required, is to supply a single power source for each color. Thus a plurality of LED strings of a single color is driven by a single power source, and the number of power sources required is reduced to the number of different colors, i.e. typically to 3. Unfortunately, since as indicated above, different LED strings of the same color may exhibit different voltage drops, such a solution further requires an active element in series with each LED string to compensate for the differing voltage drops so as to ensure an essentially equal current through each of the LED strings of the same color.
One known problem of LCD matrix displays is motion blur. One cause of motion blur is that the response time of the LCD is finite. Thus, there is a delay from the time of writing to the LCD pixel until the image changes. Furthermore, since each pixel is written once per frame, and is then held until the next frame, smooth motion is not possible. The eye notices the image being in the wrong place until the next sample, and interprets this as blur or smear.
This problem is addressed by a scanning backlight, in which the matrix display is divided into a plurality of regions, or zones, and the backlight for each zone is illuminated for a short period of time in synchronization with the writing of the image. Ideally, the backlighting for the zone is illuminated just after the pixel response time, and the illumination is held for a predetermined illumination frame time whose timing is associated with the particular zone. Thus, the display is only illuminated for a short period of time and is not senses as being in the wrong place.
An additional known problem of LCD monitors is the lack of contrast, and in particular in the presence of ambient light. An LCD monitor operates by providing two linear polarizers whose orientation in relation to each other is adjustable. If the linear polarizers are oriented orthogonally to each other, light from the backlight is prevented from being transmitted in the direction of the viewer. If the linear polarizers are aligned, the maximum amount of light is transmitted in the direction of the viewer. Unfortunately, a certain amount of light leakage occurs when the polarizers are oriented orthogonally to each other, thus reducing the overall contrast.
This problem is addressed by adding dynamic capability to the scanning backlight, the dynamic capability adjusting at least one of the overall luminance and the white point of the backlight for each zone responsive to the video signal. Thus, in the event of a dark scene, the backlight luminance is reduced thereby improving the contrast.
The addition of dynamic and scanning capability adds to complexity, and may result in certain LED strings of a particular color exhibiting a PWM duty rate different from other LED strings of the same color. Thus, each LED string controlled by a particular controller may exhibit a particular PWM duty rate unrelated to the PWM duty rate of another LED string controlled thereby.
U.S. patent application Ser. No. 11,676,313, filed Feb. 19, 2007 in the name of Korcharz et al, entitled “Voltage Controlled Backlight Driver”, the entire contents of which is incorporated by reference, is addressed to a system and method for controlling the output voltage of a power supply driving a plurality of LED strings, each of which is driven by a particular PWM signal. Such a system and method is preferably supported by a method and system for rapidly and consistently sampling the current through each of the plurality of LED strings and/or the voltage across each of the current limiters associated with the LED strings. It is to be understood that sampling the current and/or voltage, typically via an analog to digital (A/D) converter, takes a predetermined period of time, and a PWM channel, such as a LED string, is only active during a portion of a PWM period, the ratio of the active portion period to the total PWM period being denoted the PWM duty cycle.
The above mentioned application further discloses detecting an error condition of the LED strings, such as a short circuit, responsive to the sampling. Such an error detection is preferably supported by a method and system for rapidly sampling the current through each of the plurality of LED strings and/or the voltage drop across each of the current limiters associated with the LED strings when the PWM signal is active so as to rapidly detect a full or partial short circuit condition prior to burn out of one or more of the current limiters or damage to other circuit elements.